TL;DR I hope you take some extra time to frame the beta test launch with some infographics and concise language before actually going ahead with it!
I think language matters here, and you should potentially consider giving the forthcoming test a special spin - not "Open Beta", or "New Engine", but rather something that will properly communicate what we're getting and what the expectations should be, i.e., this is not the full new Engine Upgrade.
It could be "Test Drive 1", "APB Labs", etc.
The marketing could give a go at some funny graphics and ads to go with it, as long as it's made clear it's small, it's the beginning, and it's not THE engine upgrade - remember, when APB:R originally opened from Closed Beta to Open Beta, it was pretty much the same game it is now. If you use the same terminology, I think we'll see some disappointed and confused returning players.
Furthermore, the test should be clearly framed as something that has the intention of minor extension rather than major in terms of player population. We obviously want players to rush back in, but the fact of the matter is, I'd much rather keep this first test to the existing community rather than disappoint potential newcomers or returnees.
It's been proven over and over that there are legitimately tons of players willing to give APB:R another shot, but I don't think this will be the shot they should be taking. I think conveying this matter to streamers is also very important.